Examples of heat exchanger are disclosed in Patent Documents 1 and 2. As shown in FIG. 52, the heat exchanger disclosed in the Patent Document 1 includes a heat exchange coiled tube 40e placed in a housing 2e. The bottom of the space 3e surrounded by the coiled tube 40e is closed by a partition 6e. In the heat exchanger, when combustion gas is introduced from the top of the housing 2e, the combustion gas flows out from the space 3e by passing through the clearances of the coiled tube 40e and is discharged to the outside through the bottom opening of the housing 2e. On the other hand, a medium is supplied to the coiled tube 40e through one end thereof, and the medium is heated by the combustion gas. The heated medium flows out of the coiled tube 40e through the other end thereof. In the head exchanger, the coiled tube 40e comprises a single helical tube, and the structure is simple as compared with a heat exchanger utilizing a large number of finned tubes, for example. Therefore, this structure is suitable for reducing the manufacturing cost and the size of the entire heat exchanger.
As shown in FIG. 53, in the heat exchanger disclosed in the Patent Document 2, a burner 90A is arranged at a lower portion of a housing 91A, and a coiled water tube 96 is provided in the housing. The water tube 96 includes a plurality of loops 96a, and baffles 97A for preventing combustion gas from flowing into the loops 96a and baffles 97B for preventing combustion gas from flowing through the space around the loops 96a are alternately provided at or around the loops. With this arrangement, the combustion gas flows alternately inside and outside of the loops 60a of the water tube 96, whereby the amount of heat transfer from the combustion gas to the water tube 96 can be increased.
However, the above-described conventional structures have the following problems.
In the conventional structure shown in FIG. 52, combustion gas flows only in one direction i.e., from inside to outside of the coiled tube 40e through the clearances. Therefore, the amount of heat transfer is small. Moreover, since the area of the overall length of the coiled tube 40e provides a large flow path area for allowing passing of combustion gas at the same time, the combustion gas is liable to act locally on one portion of the coiled tube body 40e. Therefore, in this conventional structure, the heat exchange efficiency is low. In recent years, for the purpose of environmental protection by fuel saving, reduction of the running cost and so on, the enhancement of the heat exchange efficiency of a heat exchanger is strongly needed. As effective means to enhance the heat exchange efficiency, it may be considered to recover latent heat from combustion gas (more precisely, latent heat of water vapor in combustion gas) in addition to sensible heat. However, with the conventional structure, it is difficult to recover such latent heat.
On the other hand, the conventional structure shown in FIG. 53 has a complicated structure including the same number of baffles 97A, 97B as the loops 96a in the housing 91A. Further, the diameter of the baffles 97A, 97B is generally equal to or larger than the diameter of the loops 96a, and the size is large, whereby the manufacturing cost is increased. Further, since the baffles 97A, 97B absorb heat upon its contact with the combustion gas and hence causes loss, and the provision of a large number of baffles increases the heat capacity in the entire housing 91A. Therefore, the rise time of the temperature of water in the water tube 96 in starting the hot water supply is long, so that the ability for use in an instantaneous water heater is insufficient. Moreover, since the water tube 96 comprises a single helical tube similarly to the conventional structure shown in FIG. 52, it is difficult to obtain high heat exchange efficiency in spite of the complicated overall structure.
Patent Document 1: JP-U 61-69676
Patent Document 2: JP-A 59-66646